1. Field of the Invention
The present invention relates to a fiber reinforced plastic (FRP) substrate applied to a flexible electronic device, and more particularly, to a surface treatment method of an FRP substrate which can improve surface flatness and prevent failures due to misalignment in a process of fabricating a device.
2. Discussion of Related Art
Flexible electronic devices have been the focus of considerable attention and world-wide studied since the mid-1990s. Flexible electronic devices are applied in various fields, such as in attachable displays, advertising displays, large portable displays, and the like. The leading applications are electronic paper and organic light emitting devices (OLEDs), and a field of applications using organic thin film transistors is on the forefront of development.
As an interest in flexible electronic devices increases, research into some materials as flexible substrate is progressing. Exemplary materials used for a flexible substrate are polyether sulfone (PES), poly ethylene tertraphthalate (PET), poly ethylene naphthalate (PEN), poly imide (PI), poly carbonate (PC), and the like. These materials, as shown in the following Table 1, have a relatively high glass transition temperature (Tg) in the range of about 150-220° C., and a coefficient of thermal expansion (CTE) of about 15-70 ppm/° C. (@ 55-85° C.).
TABLE 1polymerPCPESPIFRPCTE(−55–85° C.)/ppm/° C.60–705430–6012–14Young's modulus/GPa1.72.22.528–31*GPa: Giga Pascal
However, in spite of these properties, there are many difficulties in using these materials as a substrate of a flexible electric device, because a substrate swelling, expansion, or shrinking while undergoing a photolithography process, a thermal process, a chemical process, and the like, in the manufacture of a practical electronic device.
FIG. 1 is a microscopic photograph showing a state of alignment of a mask and a PC (polycarbonate) substrate 10, using a contact aligner for forming a metal gate contact pattern. An alignment mark 11 of a mask and an alignment mark 12 formed by PR (photo-resistive) film pattern on the substrate 10 are about 15 μm out of alignment. It was confirmed that such misalignment results from expansion of the substrate 10. When a series of organic electronic devices were fabricated using a PC substrate, a problem of the substrate expanding by about 15 μm per 5 cm section was confirmed. Such expansion was seen in a PES substrate as well.
FIG. 2 is a microscopic photograph showing a state of alignment of a mask and a PES substrate 20, aligned using a contact aligner for forming a metal gate contact pattern. An alignment mark 21 of a mask and an alignment mark 22 formed by PR film pattern on the substrate 20 are about 20 μm out of alignment.
Accordingly, both PC and PES substrates have a problem of expansion by 15 μm or more per 5 cm. This problem can become quite serious when the substrates are actually used to form electronic devices. In order to form an organic flexible active-matrix OLED and electronic paper, while there is a slight variation depending on the complexity of the device to be formed, photolithography processes should be performed at least six times. However, when such misalignment happens, a device cannot be integrated, and in the case of an organic luminescent emitting display having a 5 cm panel, a part of the panel may not operate owing to misalignment.
Therefore, in order to easily form a fine pattern of a highly integrated device, a plastic substrate that does not expand, shrink or deform significantly should be used. However, plastic substrates currently being used do undergo deformation such as expansion and shrinking in a process of fabricating a device, and have very low surface smoothness, therefore making them difficult to employ in device manufacture.